The invention pertains to an absorbent structure comprising a polysaccharide-based highly absorbent material.
The invention also relates to an absorbent article, such as a diaper, an incontinence protector, a sanitary napkin, or the like, comprising the absorbent structure.
For many applications, such as in absorbent articles intended for absorption of body fluids, it has become increasingly common to use what are known as superabsorbent materials. With superabsorbent materials are meant polymers which are capable of absorbing liquid in amounts corresponding to several times the weight of the polymer and which upon absorption form a water-containing gel.
The main advantage of using superabsorbent materials in absorbent articles is that the volume of the absorbent articles can be considerably reduced in comparison to the volume of absorbent articles mainly formed from absorbent fibrous materials such as fluffed cellulose pulp, or the like. Another advantage is that superabsorbents, when compared to fibrous absorbents such as, for instance, fluffed cellulose pulp, have a higher capability of retaining liquid under pressure. Such a property is, for instance, advantageous when the absorption material is used in diapers, incontinence guards or sanitary napkins, since absorbed body fluid is retained in the absorbent article and is not squeezed out of the article, for instance when the user is sitting down.
However, a disadvantage with many of the superabsorbent materials presently being used in absorbent articles such as diapers, incontinence protectors or sanitary napkins, is that they are not produced from renewable raw materials. In order to solve this problem, it has been suggested that superabsorbents based on different types of renewable starting materials, such as polysaccharides and, in particular, starch, be used. Unfortunately, the polysaccharide-based superabsorbents which have so far been produced exhibit considerably lower absorption capacity than the commonly used polyacrylate-based superabsorbents. Further, the ability of the polysaccharide-based superabsorbents to retain absorbed liquid when the superabsorbent is subjected to load is low in comparison with polyacrylate-based superabsorbents.
In WO 95/31500, a method for producing absorbent, preferably superabsorbent, foam materials by phase separation and crosslinking of a polymer solution is described. The absorbent materials obtained exist in the form of a crosslinked open-celled polymer foam, which implies that fluid may pass between cells. By means of the described production method, it is also said to be possible to obtain biodegradeable absorbent foam materials. Preferred polymers for producing the absorbent materials which are disclosed in WO 95/31500 are hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), which are preferably crosslinked with divinyl sulphone (DVS).
The known absorbent foam materials are relatively expensive to produce and are primarily intended for medical applications, such as controlled release systems or as artificial skin and blood vessels. However, a further possible use for the described foam materials is said to be in reusable diapers or the like. The high production cost does, however, mean that the known foam materials would, in practice, not be contemplated as absorption material for absorbent articles intended for single use only.
For these reasons, there exists a demand for an improved superabsorbent material based on renewable raw materials. Accordingly, the absorption capacity for polysaccharide-based superabsorbents needs to be improved in order to make such superabsorbents an equal alternative with regard to absorbency and when compared to the superabsorbents which are commonly used today. Moreover, there exists a need for a disposable absorbent article comprising an absorbent structure with a superabsorbent material which may be produced using low-cost, readily available, renewable starting materials.
The present invention provides an absorbent structure of the kind mentioned in the introduction, and having an improved absorption capacity as compared to previously known such absorbent structures.
The absorbent structure in accordance with the invention is primarily distinguished by highly absorbent material being produced by crosslinking and desiccation of a liquid solution containing a starting material in the form of a crosslinkable polysaccharide-based polymer, wherein the starting material, after the crosslinking reaction, exists in the form of a liquid-swollen hydrogel, and wherein the crosslinked, liquid-swollen hydrogel is desiccated by extraction with a polar solvent.
A wide range of solvents may be used for the initial solution containing the polysaccharide-based polymer starting material. However, the solution containing the starting material is preferably an aqueous solution.
Surprisingly, it has been shown that by drying a crosslinked polysaccharide using a polar solvent, such as ethanol, acetone or isopropanol, a superabsorbent material can be obtained exhibiting superior absorbency when compared to a material of the same composition but dried using another method. The improved absorbency is evident both in a higher absorption capacity and in a greater ability to retain absorbed liquid even when the absorption material is subjected to pressure. The absorbency of a superabsorbent material which has been dried using a polar solvent is considerably higher than that of a corresponding superabsorbent material which has been dried using any other method, regardless of whether the absorbed liquid is water or a salt-containing solution such as urine.
When comparing electron scanning micrographs of crosslinked superabsorbent gels with the same composition, but dried in different ways, it is clearly evident that the microstructure of the dried gels, or xero-gels, show significant differences depending on the method of desiccation. Accordingly, an air-dried gel exhibits a dense, compact structure, while a gel which has been dried by solvent extraction exhibits a structure with a high degree of microporosity. Vacuum drying produces a structure exhibiting some degree of microporosity and can be said to represent a form between the Structure obtained by air-drying and the structure obtained by the solvent drying in accordance with the invention.
A probable explanation of the advantageous effect of solvent drying is that a commonly occurring phenomenon producing a dense, horny, non-absorbing structure, is avoided. This phenomenon is well known to the person skilled in the art, even though its exact mechanisms have not yet been fully explained. However, the effect is that the crosslinked gel exhibits reduced swelling capability and, thus, reduced absorption capacity. Accordingly, in comparison with conventionally dried gels, a gel which has been dried using a polar solvent exhibits a more open and flexible structure, something that affects the absorption process in a positive way.
The solvent-dried superabsorbent polymer exists in the form of a microporous gel. The superior absorption properties exhibited by the gel are believed to be the result of liquid partly being bound in the gel in a conventional manner and partly being absorbed in the microvoids in the gel. When the gel absorbs liquid, the gel swells, whereby the size of the microvoids increases and the absorption capacity of the gel is enhanced.
The starting material may comprise a polymer blend comprising an electrically charged polysaccharide-based polymer and an electrically uncharged polysaccharide-based polymer. The ratio between the charged polymer and the uncharged polymer is preferably between about 2:1 and about 4:1 and most preferably about 3:1.
A major advantage afforded by the invention is that carboxymethyl cellulose (CMC) can be used as a starting material for the production of a superabsorbent material displaying high absorption capacity and good liquid retention. The fact that CMC is produced from wood which is a renewable material source and, further, that it is readily available and comparatively low in cost, makes CMC particularly suitable for use in disposable absorbent articles. Moreover, with regard to biodegradability and compostability, CMC exhibits excellent characteristics.
However, it has been found to be less suitable to use CMC as sole starting material for the production of a superabsorbent material, since CMC tends to form intramolecular crosslinks instead of crosslinks between different molecules. An absorption material having particularly good properties may thus be obtained with a starting material comprising a mixture of CMC in the form of its sodium salt (CMCNa) and hydroxyethyl cellulose (HEC). A suitable proportion between the amount of CMCNa and HEC has thereby been found to be between about 2:1 and about 4:1 and preferably about 3:1. At a lower concentration of HEC, the resulting cross-linked gel does not exhibit sufficient gel strength. High concentrations of HEC should be avoided since the swelling capacity and, accordingly, the absorption capacity will be insufficient if the HEC concentration is too high.
Alternatively, it is possible to use combinations of other charged and uncharged polysaccharides. Some further examples of suitable charged polysaccharides are carboxymethyl starch, oxidized starch and oxidized cellulose. Suitable uncharged polysaccharides include, but are not limited to: ethylhydroxyethyl cellulose (EHEC), hydroxypropyl cellulose (HPC) and hydroxypropyl starch (HPS).
It is further possible to use pectin as starting material.
The polymer solution is preferably crosslinked with a crosslinking agent producing covalent crosslinks. Some examples of crosslinking agents of this kind are divinylsulphone (DVS), acetaldehyde, formaldehyde, glutaraldehyde, diglycidyl ether, diisocyanates, dimethyl urea, epichlorohydrin, oxalic acid, phosphoryl chloride, trimetaphosphate, trimethylomelamine, polyacrolein. Naturally, it is also possible to use ionic crosslinking or physical crosslinking such as hydrophobic/hydrophilic interactions, or the like.
A superabsorbent material of the above-described kind may be readily combined with fibres and can accordingly be mixed with absorbent fibres such as fluffed cellulose pulp, rayon, peat moss, cotton, hemp, flax, or the like, using any conventional method. Furthermore, the highly absorbent material may be mixed with non-absorbent fibres such as polyethylene, polypropylene, polyester, nylon, bicomponent fibres, or the like. Clearly, it is possible to mix different types of fibres in an absorbent fibrous structure in order to achieve an optimal combination of characteristics such as absorbency, liquid retention, shape stability, and resiliency. The fibrous structure may be bonded, for instance by the melting of thermoplastic fibres comprised in the fibrous structure, or by adding a special binding agent. In addition, the fibrous structure may have been subjected to further processing, such as compression, needling, softening, or the like.
The highly absorbent material may, of course, alternatively be placed in a layer in an absorbent body comprising further layers of fibres, nonwoven sheets, tissue paper, or the like. The highly absorbent material may be a self-sustaining layer, or may be applied onto or within a substrate. Some examples of materials which may serve as substrates are tissue sheets, foam materials, nonwoven sheets, fibrous webs, structures having pockets in which the highly absorbent material is arranged, or the like.